I had to register for a physical fitness class in college. There were several options, but a friend of mine suggested we take a Tae Kwon Do class. I registered, and he didn't. He went with ultimate frisbee or something.

In Tae Kwon Do class, another friend introduced me to my now wife. That other friend also dropped the class, and my future wife didn't have a sparring partner.

If I hadn't registered for the class...

If my friend had taken it with me and we had been sparring partners...

If the other friend hadn't dropped the class and had been my wife's sparring partner...

We ended up hanging out a lot, bonding and talking while also kicking each other.

We also lived in the same off-campus housing. My roommates had wanted to get on-campus housing, but there were no rooms with kitchens left, only dorms by the time we picked. We voted, and I chose the place with the kitchen. If I had picked the dorms...

There were hundreds of other small decisions and weird coincidences, but ultimately we fell in love and have been together for 20 years.